Apparatus for and method of reproducing an electrostatic charge pattern

ABSTRACT

An electrostatic charge pattern on an image-bearing surface is reproduced on a collector plate by means including a transfer sheet. The transfer sheet, comprising electrically insulating material, is disposed for reciprocal movement between the imagebearing surface and the collector plate. The transfer sheet is first disposed with one surface adjacent the image-bearing surface. The other surface of the transfer sheet, having a plurality of discrete dots of electrically conductive material thereon, is momentarily grounded to induce thereon an electrostatic charge whose charge density varies with the charge pattern on the image-bearing surface. The transfer sheet is then moved to the collector plate which comprises an electrically insulating plate with a plurality of electrically conductive discrete dots thereon adapted to contact the dots on the transfer sheet. The opposite surface of the collector plate is next grounded so that the electrostatic charge pattern on the transfer sheet is transferred to the collector plate.

United States Patent [1 1 Williams APPARATUS 'FoR AND METHOD or REPRODUCING AN ELECTROSTATIC CHARGE PATTERN Primary Examiner-Richard L. Moses Att0meyl-l.. Christoffersen et al.

[57 ABSTRACT An electrostatic charge pattern on an image-bearing surface is reproduced on a collector plate by means including a transfer sheet. The transfer sheet, comprising electrically insulating material, is disposed for reciprocal movement between the image-bearing surface and the collector plate. The transfer sheet is first disposed with one surface adjacent the image-bearing surface. The other surface of the transfer sheet, having a plurality of discrete dots of electrically conductive material thereon, is momentarily grounded to induce thereon an electrostatic charge whose charge density varies with the charge pattern on the image-bearing surface. The transfer sheet is then moved to the collector plate which comprises an electrically insulating plate with a plurality of electrically conductive discrete dots thereon adapted to contact the dots on the transfer sheet. The opposite surface of the collector plate is next grounded so that the electrostatic charge pattern on the transfer sheet is transferred to the collector plate.

11 Claims, 8 Drawing Figures WIIIIIUIIIA'IIIIIII A 1 I Ill l-Ill Il l Ill! I III l IIH.

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PATENTEU v 3,776,634

SHEU 2 [1F 3 APPARATUS FOR AND METHOD OF REPRODUCING AN ELECTROSTATIC CHARGE PATTERN BACKGROUND OF THE INVENTION This invention relates generally to an apparatus for and a method of reproducing an electrostatic charge pattern. More particularly, the invention relates to an apparatus for and a method of reproducing, on a collector plate, an electrostatic charge pattern conforming substantially in configuration to an electrostatic charge pattern on an image-bearing surface without degrading the original electrostatic charge pattern. The novel apparatus and method are particularly useful in the electrostatic printing art for increasing the intensity of the electrostatic charge in a reproduced charge pattern and for providing means for making multiple copies from an original electrostatic charge pattern.

It has been proposed, in the electrostatic printing art, to provide anelectrostatic charge pattern on an electrically insulating surface, either by applying the electrostatic charge pattern directly to the surface or by electrostatically charging, in darkness, the surface of an electrophotographic recording element and exposing it to a light image to be reproduced. The electrostatic charge pattern is then developed with electroscopic toner powder which is either fixed to the surface or transferred to a transfer member on which it may be fixed, by anysuitable means known in the art. When, however, the electrostatic charge pattern is thus developed, it is either destroyed or significantly degraded so that only one developed copy can be made from it. If further developed copies are desired, a new electrostatic charge pattern is usually provided for each copy. It has also been proposed to transfer an electrostatic charge pattern from one insulating surface to another insulating surface, as, for example, in US. Pat. No. 3,084,061, but the intensity of the charge of the reproduced charged pattern is no greater than that of the original charge pattern.

Under some circumstances in the electrostatic printing art, it is desirable to reduce the exposure time and- /or the total amount of light required to expose a uniformly electrostatically charged electrophotographic recording element with an image to obtain a latent electrostatic charge pattern of the image. It is believed that to obtain a high sensitivity in the electrostatic charge pattern, it is necessary to work with low electric fields. Thus, the production and development of the latent electrostatic charge pattern under these conditions becomes progressively more difiicult as the amount of electrostatic charge decreases. This is particularly true when producing electrostatic charge patterns by electroradiography, as for example, described in US. Pat. No. 2,859,352 for Electro'radiography, issued on November 4, 1958 to M. L. Sugarman, Jr., and incorporated herein by reference. In the production of an x-ray image by electroradiography, it is desirable to use the lowest amount of electromagnetic (x-ray) radiation possible to avoid the dangers to people of too much x-ray radiation.

The novel apparatus and method of the present invention provide means for reproducing an electrostatic charge pattern repeatedly from an original electrostatic charge pattern so that multiple copies can be made from the original electrostatic charge pattern without destroying or degrading it. The novel apparatus and SUMMARY OF THE INVENTION The novel apparatus for reproducing, on a collector plate, an electrostatic charge pattern conforming substantially in configuration to an electrostatic charge pattern on an image-bearing surface comprises means to dispose a transfer sheet for reciprocal motion between the image-bearing surface and one surface of the collector plate. The transfer sheet and the collector plate comprise insulating material, having adjacent surfaces with a plurality of discrete dots of electrically conductive material thereon. Means are provided to apply a source of charge migration first to the dots of the transfer sheet when the transfer sheet is adjacent the image-bearing surface, and then to the opposite surface of the collector plate when the dots of the transfer sheet and the collector plate are in contact with each other.

In one embodiment of the novel apparatus, the collector plate has a plurality of discrete dots of electrically conductive material on each of the opposite surfaces thereof.

In another embodiment of the novel apparatus, the collector plate has a plurality of dots only on the sur face adjacent the transfer sheet, and a continuous metalized layer on the opposite surface.

The novel method of reproducing, on the collector plate, an electrostatic charge pattern conforming substantially in configuration to an electrostatic charge pattern on the image-bearing surface, with the aid of the novel apparatus, comprises: (a) disposing the insulating (non-dot) surface of the transfer sheet within the area of influence of the electrostatic field of the electrostatic charge pattern on the image-bearing surface, (b) momentarily applying a source of charge migration to the dots on the opposite surface of the transfer sheet to induce thereon an electrostatic charge pattern whose charge density varies with the charge pattern on the image-bearing surface, (c) moving the transfer sheet to the collector plate so that the dots of the transfer sheet contact the dots of the collector plate, and (d) applying a source of charge migration to the opposite surface of the collector plate, whereby to transfer the electrostatic charge pattern from the transfer sheet to the collector plate.

In a preferred embodiment of the novel method, the steps (a), (b), (c), and (d) are repeated at least once, whereby to enhance the electrostatic charge pattern on the collector plate.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a front elevational view, partly in cross section and partly schematic, of the novel apparatus;

FIG. 2 is a perspective view, partly schematic, of a portion of the novel apparatus during one of the operations of the novel method;

FIGS. 3, 4, and 5 are front elevational views, partly in cross section and partly schematic, of the novel apparatus during different operations of the novel method;

FIG. 6 is an enlarged, fragmentary, plan view of a portion of the dotted surface of the transfer sheet of the novel apparatus;

FIG. 7 is an enlarged, fragmentary, cross-sectional view of the transfer sheet taken along the line 77 of 9 DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG. 1 of the drawings, there is shown an apparatus 10 for reproducing, on a collector plate 12, an electrostatic charge pattern conforming substantially in configuration to an electrostatic charge pattern on an imagebearing surface 14 of a recording element 16. The recording element 16 can be either an insulating member, such as a sheet of insulating plastic material of Mylar, for example, capable of retaining an electrostatic charge on the surface 14 thereon; or it can be an electrophotographic recording element wherein the image-bearing surface 14 is the surface of the photoeonductor, such as one comprising zinc oxide, for example, and capable of retaining an electrostatic charge pattern in darkness. A suitable recording element 16 can be one of the type described in US. Pat. No. 3,579,332 for Single Crystal Zinc Oxide and Electrophotographic Plate Made Therefrom, issued to R. Williams on May 18, 1971.

The recording element 16 is disposed in electrical contact with a metal base 18, as shown in FIG. 2. The base 18 is in the shape of a rectangular plate. The base 18 is connected to a source of reference potential, such as ground. A transfer sheet 20 is disposed for reciprocal movement between the image-bearing surface 14 and the collector plate 12 in a manner hereinafter to be described.

Referring now particularly to FIGS. 1 and 2, the rectangular base 18 is shown provided with four cylindrical posts 22, 23, 24, and 25, fixed to the base 18 near the comers thereof and extending upwardly therefrom. Electromagnets 26, 27, 28, and 29 are also fixed to the base 18 between the posts 22, 23, 24, and 25 and the corners of the rectangular base 18, respectively, for the purpose hereinafter appearing.

The collector plate 12 is a sheet of insulating material, such as the plastic material Mylar, for example, rectangular in shape, and having a uniform crosssectional thickness of between about 0.25 and 2.0 mils. The collector plate 12 is secured in a rectangular frame 30 that is formed with four holes, each adjacent a corner thereof and adapted to receive a separate one of the posts 22-25 therein, as shown in FIG. 1. Fixing means, such as screws 32, are provided to secure the frame 30 to the posts 22-25 so that the collector plate 12 is disposed parallelly to, and spaced from, the image-bearing surface 14. The space between the collector plate 12 and the image-bearing surface 14 is not critical and may be between 1 and 6 inches, for example.

The transfer sheet 20 is also rectangular and consists of electrically insulating material similar to that of the collector plate 12. The transfer sheet 20 is fixed to a rectangular frame 34 of a magnetic metal by any suit able means. The rectangular frame 34 is formed with holes adjacent its comers so that it can be slidably disposed on the posts 22-25. A separate spring 36 is disposed about each of the posts 22-25 urging the frame 34 against the frame30.

The upper surface 38 of the transfer sheet 20 has a plurality of discrete dots 40 of electrically conductive material, such as silver, chromium, or copper, for example, fixed thereto and applied by any suitable metalizing means, known in the art. Each of the dots 40 is about 0.2mm in diameter and is spaced from adjacent dots 40 by a distance of about 0.4mm between centers. The thickness of the dots 40 is not critical and can be about one micron. The lower and upper surfaces 42 and 44 of the collector plate 12 are also formed with a plurality of lower and upper dots 46 and 48, respectively, similar to the dots 40. While the dots 48 on the upper surface 44 of the collector plate 12 are preferable, a continuous layer of a metal can be used instead of the metal dots 48. The lower surface 41 of the transfer sheet is free from dots.

' The springs 36 normally urge the frame 34 away from the base 18 so that the dots 40 of the transfer sheet 20 substantially contact the lower dots 46 of the collector plate 12. Means are provided to reciprocate the transfer sheet 20 between the collector plate 12 and the image-bearing surface 14 of the recording element 16. To this end, the electromagnets 2629 are connected in circuit with a source 50 of voltage and a switch 52, as shown in FIGS. 1 and 4. When the switch 52 is open, as shown in FIG. 1, the electromagnets 26-29 are deenergized, and the transfer sheet 20 is disposed substantially against the collector plate 12. When the switch 52 is closed, as shown in FIG. 4, the metal frame 34 can be held attracted to the electromagnets 26-29 so that the lower surface 41 of the transfer sheet is either in contact with the image-bearing surface 14 or at least substantially within an area of influence of the electric field of an electrostatic charge pattern on the image-bearing surface 14.

Means are provided to apply a source of charge migration, such as an electrical connection to ground, momentarily to all of the dots 40 on the transfer sheet 20 when the transfer sheet 20 is adjacent the imagebearing surface 14, as shown in FIG. 4. To this end, a grounded, electrically conductive roller 53, as of a conductive silicon rubber composition known in the art, is rolled across all of the dots 40 in the direction of the arrow 54.

Means are provided to apply a source of charge migration to the upper dots 48 of the collector plate 12 when the dots 40 of the transfer sheet 20 are substantially in contact with the lower dots 46 of the collector plate 12, as shown in FIG. 5. To this end, a rectangular metal sheet 56 is disposed in contact with the upper dots 48, (or, alternatively, with the continuous metal layer, not shown) of the collector plate 12, as shown in FIGS. 1, 4, and 5. The metal plate 56 is connected to ground through a switch 59 to ground it, whereby to apply a source of charge migration to it. The metal plate 56 is formed with holes that receive the posts 22-25 therein, and means are provided to move an ungrounded roller 57, as shown in FIG. 5, in the direction of the arrows 58, so as to press the collector plate 12 between the grounded metal sheet 56 and the transfer sheet 20. Thus, the dots 40 of the transfer sheet contact the lower dots 46 of the collector plate 12, and the upper dots 48 (or, alternatively, the continuous metal layer, not shown) of the collector plate 12 contact the grounded metal sheet 56. 1

The novel method of reproducing, on the collector plate 12, anelectrostatic charge pattern conforming substantially in configuration to an electrostatic charge pattern on the image-bearing surface 14 of the recording element 16, will now be explained with the aid of the apparatus 10.

The novel method will be illustrated with an electrostatic charge pattern provided by electroradiography in the manner substantially described in the aforementioned U.S. Pat. No. 2,859,352. The image-bearing surface 14 of an electrophotographic recording element 16 is uniformly electrostatically charged, in darkness, by a corona discharge device 60, as, for example, described in US. Pat. No. 3,287,614 for Portable Self- Powered Corona Charging Apparatus, issued to M. M. Sowiak on Nov. 2, 1966 and herein incorporated by reference. A wire 62 of the corona discharge device 60 is connected to the negative terminal of a high-voltage (about 5,000V) source 64. The positive terminal of the voltage source 64 is grounded, as in the base 18. Under these conditions, the image-bearing surface 14 is uniformly electrostatically charged negatively, in darkness. A screen of insulating material 66, preferably containing material that fluoresces when exposed by xrays, and in common use for taking x-ray pictures, is disposed over the uniformly charged, image-bearing surface 14, as shown in FIG. 3. An object 68, such as a forearm of a person to be x-rayed, is placed on the screen 66, in darkness. The recording element 16 is now exposed by electromagnetic radiation in particular x-rays produced by a suitably energized x-ray tube 70, as shown in FIG. 3.

After the exposure, an electrostatic (negative) charge pattern of the object 68 is provided on the image-bearing surface 14 of the recording element 16. The screen 66 is now removed, and the transfer sheet 20 and the collector plate 12 are disposed over the posts 22-25, as shown in FIG. 1, in the manner described supra. In practice, it is preferable to first provide the electrostatic charge pattern on the recording element" 16 at a place remote from the apparatus 10, and then to position the recording element 16 in place in the apparatus 10.

The transfer sheet 20 is now moved within an area of influence of the electrostatic charge pattern on the image-bearing surface 14 and held there by closing the switch 52, as shown in FIG. 4. A source of charge migration is next applied to the dots 40 on the upper surface 38 of the transfer sheet 20 by rolling the grounded roller 53 over all of the dots 40, in the direction of the arrow 54. Thus, an electrostatic (positive) charge pattern is induced on the dots 40 of the transfer sheet 20 that conforms substantially in configuration to the (negative) charge pattern on the image-bearing surface 14. If the electrostatic charge pattern on the imagebearing surface 14 is negative, the electrostatic charge pattern induced on the dots 40 is positive and conforms substantially in configuration to the charge pattern on the image-bearing surface 14. It is also within the contemplation of the present invention for the aforementioned polarities to be reversed.

The switch 52 is now opened, to the position shown in FIG. 1, and the springs 36 urge the transfer sheet 20 toward the collector plate 12. To ensure that the dots 40 of the transfer sheet 20 touch the lower dots 46 of the collector plate 12, the ungrounded roller 57 is moved along the lower surface 41 of the transfer sheet 20, in the direction of the arrow 58, so that the dots 40 are pressed against the dots 46. The (positive) electrostatic charge pattern on the dots 40 is now transferred to the dots 46 where they are maintained because a source of charge migration (of electrons) is applied to the upper dots 48 (opposite the positively charged dots 46, or, alternatively, on a'metal layer, not shown) by the grounded metal sheet 56. Under these conditions, an electrostatic (negative) charge pattern is also formed on the upper dots 48 (or, alternatively, on a metal layer, not shown) of the collector plate 12 and left there when the metal sheet 56 is ungrounded and removed.

When the switch 52 is closed again, as, for example, shown in FIG. 4, the transfer sheet 20, now without any electrostatic charge, is again brought adjacent the image-bearing surface 14, leaving the collector plate 12 with a positive electrostatic charge pattern on the dots 46 and a negative electrostatic charge pattern on the dots 48. Both of the electrostatic charge patterns conform substantially in configuration to the original electrostatic charge pattern on the image-bearing surface The collector plate 12 comprises substantially a plurality of capacitors, each opposite pair of lower and upper dots 46 and 48 comprising a capacitor with the insulating sheet. In accordance with the novel method, the electrostatic charge patterns produced on the dots 46 and 48 can be enhanced, in intensity of charge, by reciprocating the transfer sheet 20 between the imagebearing surface 14 and the collector plate 12, as by opening and closing the switch 52 periodically, and by applying the source of charge migration first to the dots 40 and then to the dots 48 (or, alternatively, to a continuous metal layer, not shown), as described supra. With each cycle of reciprocation of the transfer sheet 20, electrostatic charges, conforming in configuration to the charge pattern to be reproduced, are added to the dots 46 and 48, thereby enhancing the electrostatic charge patterns on the opposite surfaces of collector plate 12. The maximum enhancement of electrostatic charge patterns on the collector plate 12 is determined by the capacity of the capacitors provided by the collector plate 12.

The electrostatic charge pattern, either positive or negative, reproduced on the collector plate 12 can be developed on the collector plate 12 in a manner well known in the art. For example, the negative electrostatic charge pattern on the upper dots 48 (or, alternatively, a continuous metal layer, not shown) on the upper surface of the collector plate 12 can be developed by cascading positively-charged electroscopic toner particles over the dots 48. Similarly, the positive electrostatic charge pattern on the lower dots 46 on the lower surface of the collector plate 12 can be developed by cascading negatively charged electroscopic toner particles over the lower dots 46. In either case, the developed charge pattern can either be fixed to the plate 12 or transferred to a transfer member, in a manner well known in the art.

Referring now to FIG. 8, there is shown means for transferring a developed but unfixed image from the collector plate 12 to a transfer member 72. The transfer member 72 may be a sheet of ordinary paper. The collector plate 12 is placed on a grounded plate 74 with the unfixed developed image to be transferred on the upper-surface of the collector plate 12. The transfer member 72 is placed in contact with the collector plate 12 so that one surface is in contact with the unfixed developed image and the other surface can be exposed to a transfer charge of an opposite polarity to that of the electroscopic toner of the image. For example, if the electroscopic toner is positive, the transfer charge will be negative so as to attract the positive toner particles to the transfer member 72. The transfer charge can be provided by a corona discharge device 76 connected in circuit with a suitable source 78 of voltage so as to provide a transfer voltage of appropriate voltage and polarity, in a manner well known in the art. The transferred image on the transfer member 72 can now be fixed thereto.

Since the original electrostatic charge pattern on the image-bearing surface 14 is not neutralized or degraded in the process of reproducing the electrostatic charge pattern on the collector plate 12, multiple copies can be made from the reproduced electrostatic charge patterns, as described. Also, since a relatively weak electrostatic charge pattern on the image-bearing surface 14 can be enhanced in its reproduced configuration on the collector plate, by the novel method, the novel apparatus and method are particularly useful in electroradiography where a minimum of x-ray exposure to a persons body is desirable.

An important advantage of the novel method is the fact that a reproduced electrostatic charge pattern on the collector plate 12 can have a greater voltage and charge density than the original electrostatic charge pattern on the image-bearing surface 14 of the recording element 16. Thus, small charge densities not ordinarily easily developable, can be substantially enhanced. The overall result of the novel method provides a greater sensitivity and a wider range of application for electrostatic photography and electrostatic copying. The energy required to produce the enhanced charge density and enhanced voltage in the reproduced electrostaticcharge pattern comes from the mechanical work done in moving the transfer sheet 20 between the image-bearing surface 14 of the recording element 16 and the collector plate 12 I claim:

l. A method of reproducing, on a collector plate, an electrostatic charge pattern conforming substantially in configuration to an electrostatic charge pattern on an image-bearing surface, said method comprising the steps of:

a. disposing a transfer sheet, comprising insulating material, within an area of influence of the electrostatic field of said electrostatic charge pattern with one surface of said transfer sheet facing toward said imagebearing surface,

b. applying momentarily a source of charge migration to all areas of the opposite surface of said transfer sheet, said opposite surface of said transfer sheet comprising a plurality of discrete dots of conductive material, whereby to induce an electrostatic charge pattern on said opposite surface of said transfer sheet conforming substantially in configuration to the charge pattern on said image-bearing surface,

. moving said transfer sheet to said collector plate, said collector plate comprising a plate of insulating material on one surface thereof, so that said dots of said transfer sheet contact said dots of said collector plate, and

d. applying a source of charge migration to the opposite surface of said collector plate, whereby to transfer the electrostatic charge pattern on said opposite surface of said transfer sheet to said one surface of said collector plate.

2. A method of reproducing, on a collector plate, an electrostatic charge pattern as described in claim 1, wherein said opposite surface of said collector plate comprises a layer of conductive material, and

the step of applying a source of charge migration to said opposite surface of said collector plate comprises momentarily contacting said layer of conductive material with said source.

3. A method of reproducing, on a collector plate, an electrostatic charge pattern as described in claim 1, wherein said collector plate comprises in addition, a plurality of discrete dots of conductive material on said opposite surface of said plate of insulating material,

and

the step of applying a source of charge migration to said opposite surface of said collector plate comprises momentarily contacting said last-mentioned plurality of dots with said source. 7

4. A method of reproducing, on a collector plate, an electrostatic charge pattern as described in claim 1, wherein the step of applying momentarily a source of charge migration to all areas of the opposite surface of said transfer sheet comprises momentarily grounding said dots on said transfer sheet.

5. A method of reproducing, on a collector plate, an electrostatic charge pattern as described in claim 1, wherein the steps (a), (b), (c), and (d) are repeated at least once, whereby to enhance the electrostatic charge pattern on said collector plate.

6. Apparatus for reproducing, on a collector plate, an electrostatic charge pattern conforming substantially in configuration to an electrostatic charge pattern on an image-bearing surface, said apparatus comprising:

means to dispose said collector plate spaced from said image-bearing surface, said collector plate comprising a plate of electrically insulating material,

a transfer sheet comprising a sheet of insulating material having a plurality of discrete dots of an electrically conductive material on one surface thereof,

means to dispose said transfer sheet for reciprocal motion between said image-bearing surface and said collector plate,

said collector plate having a plurality of discrete dots of electrically conductive material on one surface thereof and facing said one surface of said transfer sheet, and

means to apply a source of charge migration first to said dots of said transfer sheet when said transfer sheet is adjacent said image-bearing surface, and then to the opposite surface of said collector plate when said transfer sheet is adjacent said collector plate.

7. Apparatus for reproducing, on a collector plate, an

electrostatic charge pattern as described in claim 6, wherein said collector plate has a plurality of discrete dots of electrically conductive material on said opposite surface of the insulating plate thereof.

8. Apparatus for reproducing, on a collector plate, an electrostatic charge pattern as described in claim 6, wherein said means to dispose said collector plate spaced from said image-bearing surface comprises a base, a plurality of parallelly disposed posts extending from said base, and means to fix said collector plate to said posts so that said collector plate is parallel to said image-bearing surface.

9. Apparatus for reproducing, on a collector plate, an electrostatic charge pattern as described in claim 6,

wherein said collector plate has a layer of electrically conductive material on said opposite surface of the insulating plate thereof. a

10. Apparatus for reproducing, on a collector plate, an electrostatic charge pattern as described in claim 6, wherein said means to dispose said collector plate spaced from said image-bearing surface comprises a base, a plurality of parallelly disposed posts extending from said base, and means to fix said collector plate to said posts so that said collector plate is parallel to said image-bearing surface, and

said means to dispose said transfer sheet for reciprocal motion between said image-bearing surface and said collector plate comprises frame means to mount said transfer sheet slidably on said posts, spring means to urge said frame means toward the collector plate, and magnetic means to attract and hold said frame means adjacent said image-bearing surface.

11. Apparatus for reproducing, on a collector plate, an electrostatic charge pattern as described in claim 6, wherein said means to apply a source of charge migration first to said dots of said transfer sheet comprises electrically conductive, grounded roller means adapted to momentarily contact said dots of said transfer sheet.

a UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,776,634 Dated c r LI, 973

Inventor) Richard Williams It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 7, line 67, after"materia1"insert -having a plurality of discrete dots of conductive material-- Signed sealed this 30th day of July 19-71.

(SEAL) Attest:

MCCOY M. GIBSON, JR. 0. MARSHALL DANN Attesting Officer Commissioner of Patents USCOMM-DC 60376-P69 i u.s. covnuuun nnmue orrlcc nu c-au-au FORM Po-ioso (10-69) UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,776,634 Dated cem r 1973 Inventor(s) Richard. Williams It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 7, line 67, after"materia1"insert --havi ng a plurality of discrete dots of conductive material-- Signed and sealed this 30th day of July 197E.

(SEAL) Attest:

MCCOY M. GIBSON, JR. C. MARSHALL DANN Commissioner of Patents AIfiaesczIng Officer I USCOMM-DC 60376-P69 u.s. covnuunn ranmue ornc: Ion o-au-su FORM PO-IOSO (10-69) 

1. A method of reproducing, on a collector plate, an electrostatic charge pattern conforming substantially in configuration to an electrostatic charge pattern on an imagebearing surface, said method comprising the steps of: a. disposing a transfer sheet, comprising insulating material, within an area of influence of the electrostatic field of said electrostatic charge pattern with one surface of said transfer sheet facing toward said image-bearing surface, b. applying momentarily a source of charge migration to all areas of the opposite surface of said transfer sheet, said opposite surface of said transfer sheet comprising a plurality of discrete dots of conductive material, whereby to induce an electrostatic charge pattern on said opposite surface of said transfer sheet conforming substantially in configuration to the charge pattern on said image-bearing surface, c. moving said transfer sheet to said collector plate, said collector plate comprising a plate of insulating material on one surface thereof, so that said dots of said transfer sheet contact said dots of said collector plate, and d. applying a source of charge migration to the opposite surface of said collector plate, whereby to transfer the electrostatic charge pattern on said opposite surface of said transfer sheet to said one surface of said collector plate.
 2. A method of reproducing, on a collector plate, an electrostatic charge pattern as described in claim 1, wherein said opposite surface of said collector plate comprises a layer of conductive material, and the step of applying a source of charge migration to said opposite surface of said collector plate comprises momentarily contacting said layer of conductive material with said source.
 3. A method of reproducing, on a collector plate, an electrostatic charge pattern as described in claim 1, wherein said collector plate comprises in addition, a plurality of discrete dots of conductive material on said opposite surface of said plate of insulating material, and the step of applying a source of charge migration to said opposite surface of said collector plate comprises momentarily contacting said last-mentioned plurality of dots with said source.
 4. A method of reproducing, on a collector plate, an electrostatic charge pattern as described in claim 1, wherein the step of applying momentarily a source of charge migration to all areas of the opposite surface of said transfer sheet comprises momentarily grounding said dots on said transfer sheet.
 5. A method of reproducing, on a collector plate, an electrostatic charge pattern as described in claim 1, wherein the steps (a), (b), (c), and (d) are repeated at least once, whereby to enhance the electrostatic charge pattern on said collector plate.
 6. Apparatus for reproducing, on a collector plate, an electrostatic charge pattern conforming substantially in configuration to an electrostatic charge pattern on an image-bearing surface, said apparatus comprising: means to dispose said collector plate spaced from said image-bearing surface, said collector plate comprising a plate of electrically insulating material, a transfer sheet comprising a sheet of insulating material having a plurality of discrete dots of an electrically conductive material on one surface thereof, means to dispose said transfer sheet for reciprocal motion between said image-bearing surface and said collector plate, said collector plate having a plurality of discrete dots of electrically conductive material on one surface thereof and facing said one surface of said transfer sheet, and means to apply a source of charge migration first to said dots of said transfer sheet when said transfer sheet is adjacent said image-bearing surface, and then to the opposite surface of said collector plate when sAid transfer sheet is adjacent said collector plate.
 7. Apparatus for reproducing, on a collector plate, an electrostatic charge pattern as described in claim 6, wherein said collector plate has a plurality of discrete dots of electrically conductive material on said opposite surface of the insulating plate thereof.
 8. Apparatus for reproducing, on a collector plate, an electrostatic charge pattern as described in claim 6, wherein said means to dispose said collector plate spaced from said image-bearing surface comprises a base, a plurality of parallelly disposed posts extending from said base, and means to fix said collector plate to said posts so that said collector plate is parallel to said image-bearing surface.
 9. Apparatus for reproducing, on a collector plate, an electrostatic charge pattern as described in claim 6, wherein said collector plate has a layer of electrically conductive material on said opposite surface of the insulating plate thereof.
 10. Apparatus for reproducing, on a collector plate, an electrostatic charge pattern as described in claim 6, wherein said means to dispose said collector plate spaced from said image-bearing surface comprises a base, a plurality of parallelly disposed posts extending from said base, and means to fix said collector plate to said posts so that said collector plate is parallel to said image-bearing surface, and said means to dispose said transfer sheet for reciprocal motion between said image-bearing surface and said collector plate comprises frame means to mount said transfer sheet slidably on said posts, spring means to urge said frame means toward the collector plate, and magnetic means to attract and hold said frame means adjacent said image-bearing surface.
 11. Apparatus for reproducing, on a collector plate, an electrostatic charge pattern as described in claim 6, wherein said means to apply a source of charge migration first to said dots of said transfer sheet comprises electrically conductive, grounded roller means adapted to momentarily contact said dots of said transfer sheet. 